Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given (234 references).

Discotic liquid crystals: a new generation of organic semiconductors

Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photoc...

Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications.

Field-Cycling NMR Relaxometry

Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from the biological environment. (b) Gas-phase spectroscopy of clusters provides insights into fundamental interactions and into microsolvation. (c) Gas-phase data support quantum-chemical calculations. This review focuses on the current status of (poly)amino acids and DNA bases. Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment.

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Gas-phase spectroscopy of biomolecular building blocks.

Density-functional theory (DFT) and its variations provide a fruitful approach to the computational modeling of the microscopic structures and phase behavior of soft-condensed matter. The methodology takes deep root in quantum mechanics but shares a mathematical similarity with a number of classical approaches in statistical mechanics. This review discusses different strategies commonly used to formulate the free-energy functional of complex fluids for either phenomena-oriented applications or as a generic description of the thermodynamic nonideality owing to various components of intermolecular forces. We emphasize the connections among different schemes of DFT approximations, their underlying assumptions, and inherent limitations. We also address extensions of equilibrium DFT to phenomenological theories for the dynamic properties of complex fluids and for the kinetics of phase transitions. In addition, we highlight the recent literature concerning applications of DFT to diverse static and time-dependent phenomena in complex fluids.

Density-Functional Theory for Complex Fluids

Molecular chirality plays an important role in the science of liquid crystals, leading to cholesteric liquid crystal, blue phases, ferroelectric and antiferroelectric smectic phases and twist grain boundary phases. In all of these mesogens, chirality is an intrinsic property built into the chemical structure of mesogenic molecules. The study of ferroelectric liquid crystals has seen substantial experimental strides. In theoretical aspects, there has been relatively little basic work on this fascinating class of material. This review will try to present a comprehensive overview of the current status of the phase transitions in ferroelectric smectic liquid crystals. The article begins with a brief introduction about the symmetry and structure of ferroelectric mesophases. An attempt is made to identify a range of problems and related questions associated with the study of phase transitions. In the remaining parts of the article the important experimental and theoretical developments are summarized. Finally, ...

Phase transitions in ferroelectric liquid crystals

We present a simple theory of the gas-phase far-infrared collision-induced absorption spectra of two molecules with tetrahedral symmetry, methane (CH4) and tetrafluoromethane (CF4). We assume that in these molecules CIA arises through octopolar and hexadecapolar induction. By neglecting the small anisotropies in the intermolecular CH4-CH4 and CF4-CF4 potentials, we can express the spectral profiles as convolutions of single-molecule free rotation spectra with intermolecular translational components. Information theory is used to estimate the ‘least biased’ form for the latter, based on our knowledge of their spectral moments (the calculation of the zeroth through sixth moments is described in an appendix). In this way we avoid the need to make questionable assumptions about the system dynamics. The computed lineshapes are in excellent agreement with the available experimental data.

Far-infrared absorption in gaseous CH4 and CF4

Collision integrals for nonpolar polyatomic gases have been evaluated on the assumption of fixed relative orientation per collision, with a 12–12–6 potential model, for a range of reduced temperatures and shape parameters. The effect of shape of molecules on diffusion and viscosity is small but not negligible. Thermal diffusion factors are most strongly affected; the presence of interaction due to nonsphericity in the shape of molecules can wipe out the thermal‐diffusion inversion temperatures. The parameters of the spherical component of the potential have been obtained from the viscosity data for nine gases. These parameters have been used in conjunction with the second viral coefficient data to determine the quadrupole moments and shape parameters of the molecules. The quadrupole moments and shape parameters obtained are very reasonable.

Effect of shape of molecules on transport and equilibrium properties of nonpolar polyatomic gases

In the present work, the dispersion of oil palm leaf based carbonaceous quantum dots (OPL QDs) in nematic liquid crystal (NLC) E 48 eutectic mixture has been reported. The dispersed systems with concentrations 0.1, 0.2, and 0.3 wt% are designated respectively, as MIX 1, MIX 2, and MIX 3. The objective of this study is to analyze the results on the zeta potential, optical texture, dielectric constant, dielectric loss, conductivity, dielectric strength, relaxation frequency, specific power loss and total power loss of pure and OPL QDs dispersed nematic E 48 system. Zeta potential measurement has been performed in the solution state to ensure dispersion stability. The optical textures and dielectric results are recorded after filling the respective samples in sample cells. The core findings in the present study show that the zeta potential varies from − 23.43 mV to + 28.07 mV that signifies the stability of OPL QDs suspension in LCs. Specific power loss (SPL) and total power loss (TPL) are found to be least for MIX 1 which shows that the problem of high power consumption in LCDs can be resolved by dispersing a small weight percent concentration of OPL QDs in LC medium (MIX 1). Improved molecular alignment in the dispersed system has been observed from the textural study which finds its application in good contrast display devices. The color change in the aligned textures with temperature has been attributed to the birefringence change. The porous nature of carbonaceous OPL QDs has its application in supercapacitors. The benchmark results of this study highlight the effect of temperature and frequency on dielectric parameters for both planar and homeotropic state of E 48 LCs. OPL QDs dispersed system display increased conductivity for MIX 3. The decrease in the activation energy for OPL QDs dispersed system in comparison to pure LC material E 48 is a consequential result of the potential barrier change. The increment in the dielectric strength and relaxation frequency of OPL QDs dispersed system is noticed in comparison to pure E 48. These outcomes open the door for the applicability of present LCs in the field of both display and non-display devices like sensors, supercapacitors, low power consumption displays, energy conversion, and electrical storage devices as well as advanced smart systems.

Effect of carbonaceous oil palm leaf quantum dot dispersion in nematic liquid crystal on zeta potential, optical texture and dielectric properties

The second and third virial coefficient of a gas of nonspherical molecules of arbitrary symmetry have been calculated for a set of force parameters which has been found suitable to explain the dilute gas viscosity data. The influence of a large number of orientation dependent pair- and three-body nonadditive interactions has been taken into account. The relative contribution of each branch of pair and triplet interactions has been evaluated as a function of temperature. The theoretical results have been compared with the experimental data of N2, CO, CO2, CS2, C2H4 and C6H6. The agreement between theory and experiment is very satisfactory for all the systems.

Shri Singh

Papers

Phase transitions in ferroelectric liquid crystals

Far-infrared absorption in gaseous CH4 and CF4

Effect of shape of molecules on transport and equilibrium properties of nonpolar polyatomic gases

Effect of carbonaceous oil palm leaf quantum dot dispersion in nematic liquid crystal on zeta potential, optical texture and dielectric properties

Virial coefficients of the equation of state of a gas of nonspherical molecules